The formation of chromate conversion coatings on surfaces of various metals, such as zinc and cadmium, is presently the most common technique of imparting increased brightness and corrosion resistance to the metal. In a typical process, the metal work pieces are immersed in an acidic solution containing hexavalent chromium compounds, which react with the metal causing the precipitation of a complex gel-like coating or film of trivalent chromium and entrapped soluble hexavalent chromium compounds onto the metal surface. The coated work pieces are then rinsed and dried under controlled conditions.
There are several serious disadvantages common to all chromate conversion coating processes. One of these is the relatively short life of the process bath expressed in terms of unit surface area coated per unit volume of bath. The main reason for the short life is the continuous build-up in the bath of dissolved trivalent chromium resulting from the oxidation-reduction reactions that occur between the metal and the hexavalent chromium. Trivalent chromium is a contaminant in the process affecting the coating efficiency. Thus, when reduced coating activity is noted, or when the contaminants have built up to a certain predetermined level, a process solution of this type is at least partially replaced with freshly prepared solution, and ultimately completely discarded in favor of a fresh bath.
The disposal of the spent process solution is wasteful, as the solution still contains considerable quantities of hexavalent chromium. Not only does the loss of these values contribute significantly to the overall cost of the coating process, but disposal also adds to this cost in that the solutions present a substantial waste treatment problem. Hexavalent chromium is highly toxic and must be reduced to the trivalent form, e.g. by reaction with sodium hydrosulfite or sodium bisulfite, and is thereafter precipitated from solution by addition of alkalies, such as sodium carbonate or lime. After dewatering of the precipitate by settling or filtration, the concentrated sludge of trivalent chromium hydroxide must be disposed of in specially designated areas, since trivalent chromium is still too toxic to be used as landfill. Substantial waste treatment requirements of spent rinse waters are also created due to dragout of toxic chemicals from the process bath into subsequent rinse waters. Although there are integrated processes for the reoxidation and regeneration of spent chromate solutions and rinse water, the small processor usually finds that the refined and sophisticated techniques involved are neither practical nor economically feasible for solving his waste treatment problems.
In our copending application Ser. No. 955,812, filed Oct. 30, 1978, which application is incorporated into this application by reference, we have disclosed a novel nontoxic conversion coating solution which is comprised of sulfuric acid, hydrogen peroxide and a soluble silicate and optionally containing additives, e.g. certain organophosphorus compounds for further enhancement of corrosion resistance of metal surfaces treated with the solution.
Although the acidic silicate "solution" may or may not be a true solution but rather in the form of a hydrosol, for the purpose of this application, the term "solution" is intended to cover a hydrosol as well as a true solution.
In addition to the formation of conversion coatings of excellent properties, there are many other important advantages of the invention described in the aforementioned application. One of these is the extremely long life of the conversion coating solution before it is discarded in favor of a fresh solution. It has been found that the solutions are capable of treating up to approximately 185 m.sup.2 of surface area per liter, which is far superior to the typical value of approximately 20 m.sup.2 /l obtained with conventional chromate conversion coating baths.
Another and related advantage is that, apart from some build-up of dissolved metal in the solution, there are detrimental by-products forming and accumulating therein during use, as is the case with conventional chromate conversion coating solutions, in which trivalent chromium rapidly builds up.
The most important advantage, however, is the non-toxic nature of the system, which greatly facilitates waste disposal of spent solutions from the conversion coating process. Rinse waters can usually be disposed of without any treatment required. Spent conversion coating baths are merely treated with lime for neutralization and removal of dissolved metal ions and phosphorus (when organophosphorus promoters are used) as a precipitate. After settling or other separation, the liquid phase may be disposed of safely in common sewers, while the dewatered sludge mainly composed of silicate can be dumped in municipal landfill areas.
One disadvantage, however, is that the resulting conversion coatings, although bright and corrosion resistant, lack the decorative color, usually blue, which is characteristic of chromate conversion coated parts. The incorporation into the silicate conversion coating solution of conventional dyes recommended and used in the industry for coloring chromate conversion coatings failed to impart any lasting color to the coated surfaces, even at very high dye concentrations and prolonged treatment times. Attempts to use these dyes in a water solution as a post-dip treatment after formation of the conversion coatings on the work pieces also failed to impart any color.
In addition, it was found that many of the conventional metal dyes were unstable in the sulfuric acid-hydrogen peroxide-silicate environment resulting in excessive hydrogen peroxide consumption, complete loss of color or a drastic color change of the bath solution.
It is therefore an object of the present invention to provide an improved, stable conversion coating solution which imparts a lasting color to the coated surface.
Another object of the invention is a process for the formation of bright, corrosion resistant, colored conversion coatings onto metallic surfaces.
Still another object is to provide decorative bright corrosion resistant work pieces.